1. Field of the Invention and Related Art Statement
The present invention relates to an optical head for recording and/or reproducing information on and/or from an optical record medium comprising a semiconductor laser diode for emitting a diverging laser beam, a collimator lens for converting the diverging laser beam emitted by the semiconductor laser diode into a parallel laser beam, a diffraction grating for dividing the parallel laser light beam emanating from the collimator lens into a plurality of laser beams, and an objective lens for projecting said plurality of laser beams onto the optical record medium.
The above mentioned optical head using three or two laser beams have been proposed for recording and/or reproducing the information on and/or from the optical record medium such as optical disk, optical card and opto-magnetic disk. In this kind of the optical head the single laser beam emitted by the laser diode is divided into a plurality of laser beams with the aid of the diffraction grating. For instance, in U.S. Pat. No. 4,598,393, there is disclosed an optical head for use in an optical card writer and/or reader. In this known optical head, the laser light beam is divided into three light beams by means of the diffraction grating.
In the optical head of the type mentioned above, since a so-called on-axis method is adopted, a part of laser beam reflected by the optical record medium is made incident upon the semiconductor laser diode, so that the intensity of the laser beam emitted by the semiconductor laser diode might fluctuate, so that the information could not recorded correctly and S/N of a reproduced signal becomes deteriorated. In order remove such a drawback, in Japanese Patent Application Laid-open Publications Kokai Sho Nos. 61-150391 and 62-205528, there is described another known optical head using a so-called off-axis method in which the laser beams are made incident upon the record medium from directions which are inclined with respect to an optical axis of the objective lens which is perpendicular to the plane of the optical record medium.
FIG. 1 is a schematic view showing the known optical head utilizing the off-axis method. A diverging laser beam emitted from a laser emitting point 1A of a semiconductor laser diode 1 is converted by a collimator lens 2 into a parallel laser beam, and then the parallel laser beam is transmitted through a beam shaping prism 3 which serves to convert the laser beam having a elliptical cross sectional configuration into a laser beam having a substantially circular cross sectional configuration.
Then the parallel laser beam having the circular cross sectional configuration is made incident on a diffraction grating 4 which serves to divide the incident laser beam into a plurality of laser beams. Among these beams, three laser beams of zero order and .+-. first orders are selected and are made incident upon a portion 5A of an objective lens 5 which is deviated from an optical axis of the objective lens. These three laser beams emanating from the objective lens 5 are then focused onto an optical record medium 6 from directions which are inclined with respect to the optical axis which is perpendicular to a surface of the record medium.
As illustrated in FIG. 2, a central spot 7 formed by the zero order beam is projected on an information pit array 6A, and spots 8A and 8B formed by the .+-. first order laser beams are projected on guide tracks 6B and 6C, respectively, which are situated on respect sides of the information pit array 6A. In an on-track condition shown in FIG. 2, substantially half portions of the spots 8A and 8B are made incident upon the inner edges of the guide tracks 6B and 6C, respectively.
Then, three laser beams reflected by the optical record medium 6 are made incident upon a portion 5B of the objective lens 5 which is diametrically opposed to the portion 5A with respect to the optical axis, and are made incident upon a photodetector 11 by means of reflection mirror 9 and detection lens 10. As illustrated in FIG. 3, the photodetector 11 comprises light receiving regions 11A and 11B which are arranged side by side for receiving the zero order beam and light receiving regions 11C and 11D which receive the .+-. first order beams, respectively. Then an information reproducing signal is derived by a sum of outputs of the pairwised light receiving regions 11A and 11B, a focusing error signal is derived by a difference between the outputs of the pairwised light receiving regions 11A and 11B, and a tracking error signal is obtained by a difference between outputs of the light receiving regions 11C and 11D.
In the above explained known optical head using the off-axis method, the laser beams are made incident upon the optical record medium 6 from the inclined directions, so that the laser beams reflected by the optical record medium do not propagate along the same path as the incident laser beams, so that the reflected laser beams are not made incident upon the semiconductor laser diode 1.
However, in the optical head shown in FIG. 1, since the diffraction grating 4 is arranged perpendicularly to the optical axis, laser beam reflected by a surface of the diffraction grating 4 is made incident upon the light emitting point 1A of the semiconductor laser diode 1 by means of the beam shaping prism 3 and collimator lens 2. Therefore, the semiconductor laser diode 1 could not generate the laser beam stably, and the reading and writing operation could not be performed stably and accurately.
Usually in order to stabilize the output power of the semiconductor laser diode, a laser beam emitted from a rear surface backwardly is monitored and the driving current for the laser diode is adjusted such that the backward beam power becomes constant. A curve A in FIG. 4 shows the relation between the laser driving current IOP and the backward monitor current IPIN in the known optical head illustrated in FIG. 1. This curve A deviates from an ideal curve B to a great extent. Therefore, the control for the output power of the semiconductor laser diode could not be performed stably and accurately.